Electromagnetic radiation (EMR) absorbers have been used in a variety of applications. Generally speaking, applications where electromagnetic radiation absorbers are subjected to temperatures of less than about 500° F. permit the use of certain, well-known, organic materials. At temperatures above 500° Fahrenheit, organic EMR absorbers tend to fail and/or break down with a severe deterioration in their EMR absorbing capability. For application temperatures above 500° F. and less than about 1100° F., EMR absorber coatings comprising ferromagnetic particles dispersed in a dielectric are commonly used. Examples of such coating and constituents thereof are found in U.S. Pat. No. 4,731,347 (issued Mar. 15, 1988), U.S. Serial No. 855,218 (filed Mar. 21, 1986), U.S. Serial No. 891,827 (allowed Aug. 6, 1987), U.S. Serial No. 855,201 (filed Mar. 21, 1986), and U.S. Serial No. 855,199 (allowed Jan. 12, 1988). For application temperatures above about 1100° F., the ferromagnetic particle component (typically carbonyl iron and iron 10 aluminum) of such coatings oxidize rapidly—especially for ferromagnetic particle sizes of less than about 10 microns, causing substantial reduction in the absorbing capacity of the EMR absorbers.
The coefficient of thermal expansion for the composite EMR coatings is generally much less than the coefficient of thermal expansion for typical substrates (often nickel base alloys) on which the coatings are applied. Such differential thermal expansion rates result in stress fracturing of the coatings, and in some cases, spalling of the coatings from the associated substrates. Use of oxidation resistant ferromagnetic particles as a dispersed material in a vitreous matrix tends to exacerbate the problem, for example when aluminum or chromium is added to iron matrix attack increases, coating spalling increases, and radar cross-section of the substrate increases.
The EMR absorber designer, on the one hand, seeks a particle which is a good conductor and, on the other hand, is not such a good conductor as to have unreasonably small skin depths and thus require unreasonably small particle sizes. Such small skin depths and small conductive particle sizes increase the coatings' constituent cost, application cost, and accelerate particle oxidation.
At present no EMR absorber coating is known which: (1) has good absorption characteristics for application temperatures over about 1100° F.; (2) has a coefficient of thermal expansion closely approximating that of typical nickel base alloys; and (3) is applicable to substrates in reasonable thicknesses.